1. Field of the Invention
The present invention is directed to a radar system which uses two radar scans to characterize a microburst generating a windshear and, more particularly, to a system in which oriented, ordered constant false alarm rate processing reduces ground and vehicular clutter from a downward looking radar scan which is processed for a windshear hazard alert based on an upper elevation scan which determines candidate regions for windshear hazard detection processing during both landing and take off.
2. Description of the Related Art
A microburst is a meteorological phenomena of atmospheric instability which causes a windshear phenomena which is hazardous to aircraft. The instability is created by warm air at lower altitudes supporting cooler, dense air at higher altitudes. When the cooler air mass exceeds a variable threshold, warm air can no longer support the cold air and the cold air begins to penetrate and descend, often accelerating further due to evaporative cooling. A microburst derives its name from the shaft of cold air which descends and splatters or spreads out in all directions upon ground impact. This splattering is governed by fluid dynamics and the laws of conservation of mass. Behaving almost as an incompressible fluid, the downdraft to the ground is transformed into radial outflow along the ground, producing a "sombrero" envelope in space for the downdraft, outflow and vortex of the flow. For aircraft at low altitudes during landing or takeoff these windshear outflows are hazardous because they amount to a region in space where the local wind changes from a headwind to a tailwind. Aerodynamically, this is a performance decreasing windshear and will cause the aircraft to lose altitude. The hazard is further increased by the downdraft and the initial impression of a performance enhancing headwind. The severity of the hazard is balanced by the aircraft' s ability to initiate and sustain altitude performance, that is, its thrust to weight ratio. The magnitude of the hazard is measurable by the margin remaining within the aircraft's performance envelope when in a performance decreasing airstream.
The detection of hazardous weather has been performed using a double elevation or bar radar scan transmitted from the ground when a microburst is detected the pilot is alerted by the ground personnel monitoring the ground based radar. In this double bar scan relatively large vertical fan beams overlap and produce an upper beam Doppler spectrum and a lower beam Doppler spectrum which are subtracted allowing the velocity bounds of the difference Doppler spectrum to provide windspeed components at heights near the surface. However, this ground based approach is not particularly applicable to airborne systems with downlook scans because airborne radar typically employs higher frequencies, maximal directive gain and minimal antenna sidelobes. This type of double overlapping beam scan is described in U.S. Pat. No. RE 33,152. Airborne turbulence mapping systems typically involve pulse pair processing approaches in which autocorrelation is used between the echoes as typified by U.S. Pat. No. 4,835,536. Another approach is to characterize the turbulence by a variance of velocities over a range, without respect to a performance decreasing structure as described in U.S. Pat. No. 4,223,309. Neither of these approaches is suitable for a downlook system for detecting windshear.
To effectively avoid windshear hazards, pilots need an airborne windshear hazard alert system which looks at the path to be taken by the airplane and alerts the pilot in sufficient time to abort a landing or takeoff or to allow the pilot to compensate for the magnitude of the performance decreasing changes caused by the microburst.